For electrical and electronic devices, typically electrical connections are made through the expedient of a lead frame or other copper containing structures. For example, in the fabrication of integrated circuits, a silicon body having electronic circuitry is connected to a metal e.g. copper, lead frame such as shown in FIG. 1 at 2 with the chip positioned at 3 and connections between bonding pads on the chip and the lead frame shown at 4. After the chip is bonded to the lead frame, the chip is encapsulated typically in a polymer composition. The strip, 6, in FIG. 1 is removed from the lead frame in a process generally denominated trimming. Thus, after trimming, the individual leads, 7, are no longer mechanically connected on one end. The leads are then bent to facilitate connection to other electronic or electrical bodies such as a circuit board. Generally the bending involves the formation of at least one curve such as shown in FIG. 2 for typical interconnection of an integrated circuit with a circuit board.
For many applications, the interconnection between the lead frame and another electronic or electrical entity is formed using a lead/tin solder alloy. However, such alloy does not readily wet copper. Therefore, the copper leads are typically coated, e.g. plated, with a layer of tin to enhance wetting of the leads by solders before trimming. Although the tin layer does in fact facilitate wetting of the copper leads, other problems are generated. In particular, there is a tendency to form long needle-like tin structures generally denominated whiskers. These structures are usually from 20 to 100 μm in length and can grow to as long as 1 mm or more. (The whiskers are most often single crystal structures, but multi-crystal whiskers are also possible.) The exact interaction between the copper and tin producing such crystallites is not precisely known. It has been postulated that copper and tin form an intermetallic material in a manner that leads to regions of excess tin. These regions, it is contemplated, are under compressive stress, particularly at the curved sections of the lead frame after bending. The combination of excess tin and compressive stress enhances the tendency to form whisker structures. The occurrence of, and thus the problem associated with, whiskers are exacerbated because they also form when Sn is plated on brass, alloy 42 and other commonly used electronic interconnect metallization structures.
Decades ago, it was found that if, elemental lead (Pb), is added to the tin coating, whisker formation is essentially eliminated. Thus the issue of whiskers has not imposed reliability risks on electronic devices with Pb-doped tin plated leads. However, impending legislation particularly in European countries prohibits the use of lead for many applications including some involving electronic and electrical devices. Thus, there has been a substantial impetus to remove lead from the tin coating. Such removal has the potential for renewing whisker formation as an issue to be considered.
For similar reasons, use of lead-free solder is also being promoted. Such solders melt generally at temperatures above 217° C., and in application, for process control reasons, are typically used at temperatures above 240° C. Since such temperatures exceed the melting point of tin (approximately 232° C.), concerns about tin whiskers have been mitigated since such whiskers are melted during the soldering process. Accordingly, it would appear that the difficulties associated with whiskers such as inadvertent shorting of lead frames or blocking of optical paths for electro-optic devices need not be a substantial concern.